This invention relates to silicon carbide-based microelectronic devices and fabrication methods therefor, and more particularly to silicon carbide-based light emitting devices such as Light Emitting Diodes (LEDs) and laser diodes and fabrication methods therefor.
Light emitting diodes are widely used in consumer and commercial applications. As is well known to those having skill in the art, a light emitting diode generally includes a diode region on a microelectronic substrate. The microelectronic substrate may comprise, for example, silicon, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new display applications, and may place LEDs in a position to compete with the well entrenched incandescent and fluorescent lamps.
In fabricating light emitting devices, such as LEDs and/or laser diodes, using silicon carbide, it may be desirable to provide a reflective ohmic contact to the silicon carbide, and more specifically to n-type silicon carbide. These reflective ohmic contacts should be simple to fabricate using conventional microelectronic fabrication techniques, and should provide low ohmic losses and/or high reflectivity. These contacts also should be amenable to wire bonding and/or submount bonding thereto.
Reflective ohmic contacts for silicon carbide and, in some embodiments, for n-type silicon carbide, according to some embodiments of the present invention, include a layer consisting essentially of nickel on the silicon carbide. The layer consisting essentially of nickel is configured to provide an ohmic contact to the silicon carbide, and to allow transmission therethrough of optical radiation that emerges from the silicon carbide. It will be understood that, as used herein, the layer consisting essentially of nickel contains substantially only elemental nickel, and does not contain substantial amounts of nickel alloys, nickel oxides and/or other nickel compounds, but may contain insubstantial amounts of impurities ordinarily associated with nickel, including insubstantial amounts of the above-described alloys or compounds and may also contain insubstantial or substantial amounts of materials that do not materially affect the basic and novel characteristics of the elemental nickel as an ohmic contact to the silicon carbide that also allow transmission therethrough of optical radiation that emerges from the silicon carbide. Reflective ohmic contacts according to embodiments of the present invention also include a reflector layer on the layer consisting essentially of nickel, opposite the silicon carbide, a barrier layer on the reflector layer opposite the layer consisting essentially of nickel, and a bonding layer on the barrier layer opposite the reflector layer. It has been found, according to some embodiments of the present invention, that the layer consisting essentially of nickel and the reflector layer thereon can provide a reflective ohmic contact for silicon carbide that can have low ohmic losses and/or high reflectivity.
In other embodiments of the present invention, the layer consisting essentially of nickel is sufficiently thin to allow transmission therethrough of substantially all optical radiation that emerges from the silicon carbide. In yet other embodiments, the reflector layer is sufficiently thick to reflect substantially all optical radiation that emerges from the layer consisting essentially of nickel. Moreover, in other embodiments of the present invention, the silicon carbide includes a surface and a layer consisting essentially of nickel covers the surface. By covering the surface, adhesion of the nickel to the silicon carbide may be enhanced, and reflectivity of substantially all of the optical radiation that emerges from the silicon carbide may be provided. In still other embodiments, the layer consisting essentially of nickel covers only a portion of the surface and/or may be patterned, for example to form a grid.
Other embodiments of the present invention provide the reflective ohmic contact on the first face of a silicon carbide substrate and a light emitting region on a second face of the silicon carbide substrate, to provide a light emitting element such as an LED or a laser. Moreover, in other embodiments, a mounting assembly is provided on the bonding layer opposite the barrier layer. In yet other embodiments, a wire bond is provided to the bonding layer. Other external elements also may be bonded to the bonding layer.
In some embodiments of the present invention, the layer consisting essentially of nickel is a layer consisting of unannealed nickel. Moreover, in some embodiments, the layer consisting essentially of nickel is between about 15 xc3x85 and about 100 xc3x85 thick. In other embodiments, the layer consisting essentially of nickel is between about 15 xc3x85 and about 25 xc3x85 thick. In still other embodiments, the layer consisting essentially of nickel is about 15 xc3x85 thick and, in yet other embodiments, the layer consisting essentially of nickel is about 25 xc3x85 thick.
In some embodiments of the invention, the reflector layer comprises silver and/or aluminum. In some embodiments of the present invention, this layer is between about 700 xc3x85 and about 2 xcexcm thick. In other embodiments, this layer is at least about 700 xc3x85 thick. In still other embodiments, this layer is about 1000 xc3x85 thick.
In some embodiments of the present invention, the barrier layer comprises platinum. In some embodiments, this layer is between about 250 xc3x85 and about 1 xcexcm thick. In other embodiments, this layer is at least about 250 xc3x85 thick. In other embodiments, this layer is about 500 xc3x85 thick and, in still other embodiments, this layer is about 1000 xc3x85 thick.
In some embodiments of the invention, the bonding layer comprises gold. In some embodiments, this layer is between about 250 xc3x85 and about 1 xcexcm thick. In other embodiments, this layer is at least about 250 xc3x85 thick. In other embodiments, this layer is about 500 xc3x85 thick and, in still other embodiments, this layer is about 1 xcexcm thick.
Light emitting elements such as light emitting diodes may be fabricated, according to some embodiments of the present invention, by depositing a first layer consisting essentially of nickel on a first face of a silicon carbide substrate that includes a diode region on a second face thereof. A second layer comprising silver and/or aluminum is deposited on the first layer opposite the first face. A third layer comprising platinum is deposited on the second layer opposite the first layer. A fourth layer comprising gold is deposited on the third layer opposite the second layer. The fourth layer is bonded to an external element such as a mounting assembly, submount and/or wire. Annealing is not performed during the deposition of the first layer, between the deposition of the first layer and the deposition of the second layer, between the deposition of the second layer and the deposition of the third layer, between the deposition of the third layer and the deposition of the fourth layer, or between the deposition of the fourth layer and the bonding of the fourth layer. In other embodiments, patterning also is not performed during the deposition of a first layer, between the deposition of a first layer and the deposition of a second layer, between the deposition of a second layer and the deposition of a third layer, between the deposition of a third layer and the deposition of a fourth layer, between the deposition of a fourth layer and the bonding the fourth layer and during the bonding the fourth layer. In yet other embodiments, patterning may be performed in at least one of these operations. Moreover, in other embodiments, all of the above-described deposition steps are performed at room temperature. Accordingly, some embodiments of the present invention may provide ease of fabrication by eliminating some patterning steps and/or not using high temperature annealing during fabrication of the contact.